By definition, a parasite is an organism which lives on, and typically at the expense of, another organism called the "host." Parasitic organisms are found in many different phyla of the animal world, and hosts are similarly found in many different phyla of the animal world. The greatest number of parasites are members of the phyla
Platyhelminthes, the Classes Trematoda and Cestoidea;
Nematoda; and
Arthropoda, the Classes Arachnida and Insecta.
The Platyhelminthes and Nematoda are informally referred to as "helminths." These species typically live in the intestinal tract, or in some cases other internal organs, of the host; these species are often called "endoparasites." The arthropod parasites are often called "ectoparasites" because they typically attack the host on its external ("ecto") surface. Agents which control both internal and external parasites are sometimes referred to as "endectocides."
The hosts are typically vertebrates. Human beings are of course potential hosts for many parasites, and in tropical areas and in areas with minimal sanitation, parasitic infections are a regular part of medical practice. Also highly subject to attack by parasites are the numerous livestock animals such as cattle, sheep, pigs, goats, chickens, turkeys, ducks, geese, and the like. Likewise, horses are subject to parasitic attack, as are mink and other animals grown for their fur; and rats, mice and other animals used in laboratory and research settings. Companion animals such as dogs and cats are highly subject to attack by parasites, and because of their close relationship with humans, such parasitism poses problems for the humans with whom they are associated. Fish and other animals grown in aquaculture are also subject to parasitic attack. In short, parasitism involves essentially the whole range of vertebrate animals. Further, parasitic organisms have evolved many quite specific modes of interaction with their hosts, which makes them relatively inaccessible to control. Many of them live internally within the host; and in this and other ways, they have ingeniously avoided control.
The economic toll from parasite infestation is staggering. In the livestock realm, animals suffer reduced feed efficiency and growth rates. Milk and wool production suffer; and there is damage to fleece, hides, and pelts. Animals are rendered susceptible to secondary microbiological infections and to further parasite attack. Not infrequently, parasite attack is fatal. It also causes discomfort even when it is not severely detrimental to the host. Although a number of parasiticides are in use, they suffer from a variety of problems, including a limited spectrum of activity, the need for repeated treatment, and, in many instances, resistance by parasites. Therefore, there is a critical need for new parasiticides.
The present invention provides a new tool in the control of parasitic organisms, especially those "hematophagous" organisms which attack their hosts by ingesting blood. By "ingesting" is meant not only those parasites which pierce and suck the blood from a circulatory system, but also those parasites, typically arthropods, which consume tissue of the host and thereby inevitably consume blood. This new tool is a class of compounds known as formazans. Many of the compounds are old, see U.S. Pat. No. 4,069,320. This patent describes the subject compounds as useful in controlling insects, and as noted above, there are parasitic organisms among the insects. However, the teaching of U.S. Pat. No. 4,069,320 is that insects can be controlled by contact, a technique which, however successful, is inherently impractical or impossible as a tool to control blood-ingesting parasitic organisms. Only a method of achieving systemic control via the circulatory system of the host can provide truly effective control of blood-ingesting parasitic organisms, and there is nothing in U.S. Pat. No. 4,069,320, which suggests or teaches any way of providing such systemic control. Thus, the present discovery unexpectedly provides a new tool in the control of parasitic organisms.